Effect of dietary protein on body composition in adolescents and older adults

Problem: Childhood and older adulthood represent critical periods for changes in body composition. Acquisition of excess body fat and loss of lean mass are linked with numerous co-morbidities and disabilities. This study explores the role of dietary protein as a modifiable determinant of body composition at different ages. Methods: Prospective data from two studies were used: 9-10 year-old girls n=2330 followed for 10 years in the National Growth and Health Study and middle-aged-older adults n=1490 followed up to 8 years in the Framingham Offspring Study. Diet was assessed using 3-day records. Body composition was assessed using body mass index BMI, waist size an anthropometric estimate of central adiposity, and bioelectrical impedance to estimate skeletal muscle mass SMM and percent body fat %BF. Sarcopenia was defined as two SDs below mean SMM from young referent population. To avoid confounding of dietary protein by baseline weight, intake was expressed in two ways: per kilogram kg of nearest ideal body weight IBW and weight-adjusted protein residuals.Results: After adjusting for age, sex, height, socio-economic status, activity, smoking, energy intake, percent energy from fat and carbohydrates, increasing quintiles of protein intake using weight-adjusted residuals during early-mid-adolescence led to significantly lower %BF, BMI, and waist p: <0.0001, 0.0003, 0.0025 respectively. With increasing quintiles of dietary protein, SMM increased linearly p-trend: <0.0001. In older adults, higher total, animal, and plant protein intakes led to statistically significant 30%, 19%, and 37% reductions in risk of central adiposity, respectively. Risk of obesity was reduced by 35% 95% CI: 0.43-1.00, 21% 95% CI: 0.55-1.15 and 37% 95% CI: 0.42-0.97, respectively. Higher protein intakes led to nearly a 50% statistically significant reduction in risk of sarcopenia compared to the lowest tertile. Similar decreases in risk of obesity, central adiposity, and sarcopenia were observed with the clinically relevant measure of protein-kg IBW.Conclusion: Adolescent girls consuming more protein had a lower BMI and waist size and more SMM by late adolescence. Older adults consuming more protein had lower levels of body fat and a lower risk of developing sarcopenia over 8 years.